The present invention relates to an aluminum alloy for the substrate of a magnetic disk and, in particular, to the improvement in the adhesion of electroless plating being the primer treatment for the covering with magnetic substance and the surface smoothness.
For a magnetic disk used for the recording devices of electronic computer, one wherein the surface of substrate comprising of aluminum alloy is covered with magnetic substance is used generally. Such magnetic disk is manufactured such that, after the substrate was processed to a predetermined thickness and the surface was polished mirror-like a mixture of the powder of magnetic substance with the powder of resin is coated, and then the film of magnetic substance is formed by the heat treatment.
Recently, it has become necessary for the magnetic disk to have a large capacity and a high density. As a result, the magnetized area per one bit has been made increasingly minute in the magnetic disk and, at the same time, it has become necessary also to decrease the clearance between the magnetic head and the magnetic disk resulting in the requirement for the thinning and the improvement in the abrasion resistance of the film of magnetic substance. For this reason, the magnetic disk is proposed, which is manufactured such that, after the substrate has been processed to a predetermined thickness and the surface was finished mirror-like, a hard nonmagnetic metal, for example, Ni-P alloy is plated electrolessly as the primer treatment for the covering with magnetic substance, and then the magnetic substance, for example, Co-Ni-P alloy is covered by sputtering or plating.
For the substrate of such magnetic disk, the following characteristics are required.
(1) To be nonthermal treatment type and to have sufficient strength possible to withstand various processings and high-speed rotation at the time of the use.
(2) To be light in weight and to be obtained excellent mirror face by polishing without appearing the surface defects such as pits.
(3) To be excellent in the adhesion of electroless plating being the primer treatment and in the surface smoothness without the defects such as pits.
As the substrate for magnetic disk satisfying the characteristics as these, JIS A5086 alloy (Mg 3.5-4.5 wt %, Fe.ltoreq.0.50 wt %, Si.ltoreq.0.40 wt %, Mn 0.20-0.7 wt %, Cr 0.05-0.25 wt %, Cu.ltoreq.0.10 wt %, Ti.ltoreq.0.15 wt %, Zn.ltoreq.0.25 wt % and the remainder Al) or the alloy having made the quantity of intermetallic compounds formed in the matrix less by regulating Fe, Si, etc. which are impurities in JIS A5086 is used.
However, since, with the substrate comprising the JIS A5086 alloy aforementioned, the adhesion of electroless plating being the primer treatment for the covering with magnetic substance is inferior, there has been a problem that the film of electroless plating is peeled off during the covering process with magnetic substance or the use. Moreover, the surface smoothness after the electroless plating could not be said also to be sufficient.
Namely, the intermetallic compounds drop out at the time of zincate treatment to form the pits. These pits may disappear by subsequent polishing in many times if the thickness of electroless plating is as thick as about 20 .mu.m. But, the recent trend is to make the thickness of plating thin, so that even with aluminum alloy, the pits having not occurred by the polishing after plating hitherto, there has become to happen such cases as the pits occur because of the thinning of film. Moreover, the plates of aluminum alloy are stamped out in a predetermined size and planed and polished thereafter. At that time, there may be cases when the intermetallic compounds drop out to form pit defects. Based on these facts, it is required strongly to decrease the number of the intermetallic compounds in aluminum alloy and to make also the size of them small.